The invention relates to a method wherein the material is x-rayed by a primary beam that is diffracted at the material and an apparatus for determining the material of a detected item in an object.
To assure safety in situations such as air travel, it is necessary to check luggage (object) with travel items (items), particularly for explosive substances or agents, by employing the most modern technical equipment.
A useful technique for checking for explosives is X-ray diffraction, in which X-rays scattered at the crystal structure of an item are measured and compared to the characteristic energy spectra of different explosives, for example. The spectra can provide an indication of the presence of an explosive, and can thereby provide information about the nature of an explosive material in the object.
Apparatuses and methods that operate according to this principle are known from, for example, DE 195 10 168 A1, EP 0 354 045 A2and U.S. Pat. No. 4,956,856. A drawback of these methods is that absorption of the X-rays by, for example, the object and item affects the X-ray diffraction spectrum. Consequently, if spectral information is missing due to absorption, inaccurate conclusions may be drawn regarding the type of material and the identification of the substances examined.
It is an object of the invention to provide a method and an apparatus of the type originally mentioned above, with which materials of an item can be identified unambiguously.
The above object generally is achieved according to the first aspect of the invention by a method for determining the material of a detected item in an object that comprises the steps of irradiating the material with a primary X-ray beam; measuring a diffraction spectrum of the material, with the spectrum comprising X-rays of the primary beam diffracted from the material; measuring the X-ray absorption of the material and an average atomic number of the material by measuring X-rays of the primary beam transmitted through the material; and comparing the measured diffraction spectrum and measured average atomic number of the material to known diffraction spectra and known average atomic numbers of known materials to determine the material.
The apparatus for determining the material of a detected item in an object comprises a diffraction apparatus and a computer connected thereto, wherein the diffraction apparatus comprises an x-ray source and a collimator/detector arrangement, including a detector located behind a collimator, with the detector comprising an X-ray sensitive surface oriented toward the collimator. The collimator defines a central, blind-bore opening and at least one conically expanding slot, with the at least one conically expanding slot defining a predetermined angle and being oriented toward the X-ray sensitive surface of the detector. The central opening is closed to the detector and has first and second detection devices mounted therein, with the first and second detection devices being connected to the computer detecting relatively lower and relatively higher energy X-rays, respectively, and being spaced in the central opening with the second detection device located behind the first detection device.
The concept underlying the invention is to obtain additional information about the absorption behavior of the materials from the central X-ray beam during a diffraction measurement, and to make this information available along with the diffraction spectrum for evaluation and identification of the material.
The diffraction apparatus which as indicated above, generally comprises a collimator/detector arrangement and an X-ray source for generating a central beam or primary beam that is aimed at the arrangement. According to the invention, the collimator of the collimator/detector arrangement has a central, blind-bore open to and facing the X-ray source, in which first and second detector devices are arranged, with the first and second detector devices being spatially separate from each other and disposed one behind the other. The detector device that is impacted first by the primary beam is designed as a detector for relatively lower X-ray energies, and the device impacted second is designed as a detector for relatively higher X-ray energies. In a known manner, an average atomic number (ordinal number) of the material of the item located in the primary beam is additionally determined.
The diffraction apparatus is preferably mounted adjustably in an X-ray testing machine, with the collimator/detector arrangement furthermore being adjustable in height relative to the X-ray source.
The collimator/detector arrangement further comprises a circular-slot collimator having an energy-sensitive detector behind it.
The invention is de-scribed below in detail by way of an embodiment illustrated in the drawings.